Cyclic anhydrides are produced commercially by the oxidation of an appropriate hydrocarbon in the vapor phase over a suitable catalyst. For example, maleic anhydride is produced commercially by the vapor phase oxidation of benzene or straight-chain C.sub.4 hydrocarbons (hydrocarbons containing four carbon atoms), such as n-butene or butadiene, with an oxygen-containing gas, over a vanadium-phosphorus oxide catalyst. Air is generally used as the oxygen-containing gas, because of its low cost and ready availability. The reaction can be carried out in any suitable reactor, such as a fixed bed, fluidized bed, or transport bed reactor, and it produces the anhydride, and generally carbon monoxide (CO), carbon dioxide (CO.sub.2), water, and smaller amounts of other partially oxidized by-products. The reaction equipment train generally consists of a reactor, in which the anhydride is produced, a scrubber, in which the maleic anhydride is scrubbed from the reactor effluent gases by means of water or other solvent for the anhydride, and means for further treating the scrubbed effluent gases.
In the past it was common to practice the above-described process on a single pass basis with the conversion of hydrocarbon to the desired anhydride product being maximized. This resulted in a low overall efficiency, since the selectivity to maleic anhydride was below the maximum. Consequently, the scrubber effluent gas contained considerable amounts of CO and CO.sub.2, in addition to unreacted hydrocarbon. These products were usually incinerated, so that the only return realized from them was heat value. In later processes a portion of the scrubber effluent gas was recycled, the conversion of the hydrocarbon feedstock was lowered and the selectivity of hydrocarbon conversion to maleic anhydride was increased. The remainder of the effluent was purged from the system to prevent the build-up of CO, CO.sub.2 and nitrogen (introduced into the system when air is used as the source of oxygen). These improvements resulted in a reduced "per pass" conversion, but the overall efficiency of the process was increased.
Federal Republic of Germany (FRG) Patent Application Disclosure 25 44 972 discloses a maleic anhydride manufacturing process in which the reactor feed comprises C.sub.4 hydrocarbons, air, CO and CO.sub.2. In the process of this patent, maleic anhydride is recovered from the reactor effluent gas stream and a portion of the remaining stream is recycled. This patent also teaches recovering butane by temperature swing adsorption form the non-recycled gas stream and recycling the recovered butane to the reactor.
U.S. Pat. No. 4,352,755 discloses a recycle process for the vapor phase manufacture of maleic anhydride by reacting a straight-chain C.sub.4 hydrocarbon with oxygen in the presence of CO.sub.2. In the process disclosed in this patent the gaseous mixture may contain up to 30 volume percent of carbon dioxide as the inert diluent and contains at least 25 volume percent C.sub.4 hydrocarbon.
Recycling a portion of the effluent gas from gas phase anhydride reactors increases the capital costs compared to single pass processes since the size of the reactor and associated equipment must be increased to handle the increased volumes of CO, CO.sub.2 and nitrogen resulting from the recycling step. The problem is intensified when low heat capacity gases such as nitrogen are used as diluents because greater gas flows are necessary to provide adequate heat removal to prevent overheating.
Another problem associated with the gas phase production of an anhydride by the oxidation of hydrocarbons with an oxygen-containing gas is that since the reaction is carried out at elevated temperatures, there is an ever-present danger of a fire or an explosion in the reactor or the equipment or pipelines associated with the reactor. Accordingly, the concentrations of the reactants in the system are maintained such that the mixture is kept outside of the flammability range. Although nitrogen serves to reduce the flammable mixture range when air is used as the source of oxygen for the reaction, the flammable mixture range for hydrocarbon-air mixtures is still quite broad. Consequently, it has been customary to operate gas phase anhydride reactors at low hydrocarbon levels so that the reaction mixture will remain outside of the flammable range.
U.S. Pat. No. 3,904,652 teaches a gas phase maleic anhydride manufacturing process in which oxygen is used as the oxidizing gas and an inert gas, such as nitrogen, argon, helium or a lower hydrocarbon is fed into the reactor with the n-butane and oxygen, the inert gas serving as a diluent to reduce the concentrations of oxygen and butane in the reactor to below the point at which they form a flammable mixture. In the disclosed process, a portion of the gaseous effluent, which contains, in addition to butane, carbon monoxide, carbon dioxide and the inert gas, is recycled. One of the disadvantages of a process such as the one disclosed in this patent is that recycling carbon monoxide increases the fire and explosion hazard because carbon monoxide itself is highly flammable.
Because of ever increasing safety concerns and energy costs, there are continuing efforts by industry to make chemical processes involving oxygen and flammable compounds less hazardous and more economical to operate.